Although almost any measurements made at Mars provide information that is pertinent to astrobiology, the value of having an astrobiology science strategy is in being able to prioritize the possible measurements and the missions that can make them in order to provide useful scientific guidance.
The approach outlined in NASA’s 1995 report An Exobiological Strategy for Mars Exploration involved missions of increasing capability and focused on astrobiology-related issues. In the intervening decade, NASA has carried out a program of orbital reconnaissance and in situ analysis to understand the role of water. Similarly, missions are in development (e.g., Phoenix and Mars Science Laboratory) that will explore the chemistry and the biological potential of the surface in detail. With this information in hand, and following that approach, analysis of samples returned from Mars to Earth will yield the greatest increase in our understanding of Mars and thus support for addressing astrobiology science goals as well as science goals related to other aspects of Mars. A commitment to carrying out a Mars sample-return mission is necessary to ensure that such a mission does not continue to be pushed farther into the future.
Samples collected for return to Earth should include a well-chosen suite of materials collected from a diverse set of locations by a capable rover, and should include both weathered and unweathered materials with minimal thermal and shock histories. Given the MER experience and current understanding of the nature of materials on the martian surface, a “grab sample” obtained from a stationary lander is not likely to be sufficient to provide the necessary data.
Finding. Sample return should be seen as a program that NASA and the Mars science community have already embarked upon rather than as a single, highly complex, costly, and risky mission that is to occur at some future time.
As an example, the committee notes that technology development pertinent to sample return (e.g., sample handling and packaging, Mars ascent vehicle, precision landing, planetary protection, and so on) has been ongoing but needs to move forward in earnest immediately so that the necessary technology will be available in a timely manner. There is a significant heritage from ongoing Mars missions (such as MER and MSL) that will carry over to the implementation of Mars Sample Return; from sample-collection missions such as Stardust and Genesis; and from the heritage of sample planning and analysis conducted over the last two decades.
Programmatically, sample return should be phased over multiple launch opportunities. A first phase could involve caching samples on Mars; a second phase, putting samples into orbit; and a third phase, returning samples to Earth. This approach would also allow some independent science investigations at each phase that would continue to engage the science community and the public, and it would increase resilience of the program in the face of the failure or delay of individual spacecraft missions.
However, the program’s emphasis should be on sample return. Missions subsequent to MSL should emphasize science, technology, and programmatic issues that lead directly to sample return, and return of samples to Earth should be carried out at the earliest opportunity.
Recommendation. The highest-priority science objective for Mars exploration must be the analysis of a diverse suite of appropriate samples returned from carefully selected regions on Mars.
As part of an effective sample-return strategy, sample caching could be carried out by each surface mission, utilizing a minimalist approach so as not to make sample caching a cost- or technology-driver. That is, caching should not be made so complicated as to preclude actually carrying it out. Because researchers cannot predict with confidence which landing sites might provide astrobiologically interesting samples, samples should be cached from all visited sites. This strategy should allow collection of diverse samples and mitigate the costs of sample-return missions. It is, of course, dependent on the development of a precision landing capability.
Recommendation. If it is not feasible to proceed directly toward sample return, then a more gradual approach should be implemented that involves sample caching on all surface missions that follow the Mars Science Laboratory, in a way that would prepare for a relatively early return of samples to Earth.